The present invention relates generally to carbon nanotubes and more particularly to selective growth of carbon nanotubes on template materials.
It is likely that future devices containing organized structures of various functional materials with new properties will be built from nanoscale building blocks. These nanoscale building blocks can be produced by a variety of synthesis routes. The novel properties of the nanoscale building blocks arising from their low dimensions are known for a wide range of materials.
Carbon nanotubes are a nanostructured material which promises to have a wide range of applications. However, the present techniques used to controllably build organized architectures of nanotubes with predetermined orientations have several deficiencies. For example, vertically aligned nanotubes has been fabricated on catalyst printed planar substrates by chemical vapor deposition. See, for example, W. Z. Li, et al. Science 274, 1701 (1996); R. Sen, A. Govindaraj, C. N. R. Rao, Chem. Phys. Lett. 267, 276 (1997); M. Terrones, et al. Nature 388, 52 (1997); Z. F. Ren, et al. Science 282, 1105 (1998); S. S. Fan, et al. Science 283, 512 (1999); H. Kind, et al. Adv. Mater. 11, 1285 (1999); R. R. Schlittler, et al. Science 292, 1136 (2001) and L. Dai, A. W. H. Mau, J. Phys. Chem. B 104, 1891 (2000). However, this technique requires deposition and patterning, usually in separate processing steps, of catalyst material, typically in nanoparticle assemblies or thin film forms, which complicates the nanotube fabrication method. This also does not allow growth of nanotubes in more than one preselected orientation at different locations in a controllable fashion.
While growth of vertically aligned nanotubes on planar substrates by CVD has been reported extensively, obtaining nanotubes that are exclusively oriented parallel to the substrate in predetermined orientations has been more difficult. Suspended nanotubes across elevated structures have been produced recently by several different methods. One method involves adjusting the gas flow during CVD. See N. R. Franklin, H. Dai, Adv. Mater. 2000, 12, 890; N. R. Franklin, Q. Wang, T. W. Tombler, A. Javey, M. Shim, H. Dai, Appl. Phys. Lett. 2002, 81, 913; and Y. Homma, Y. Kobayashi, T. Ogino, T. Yamashita, Appl. Phys. Lett. 2002, 81, 2261. Another method involves applying an electrical field during CVD. See Y. Zhang, A. Chang, J. Cao, Q. Wang, W. Kim, Y. Li, N. Morris, E. Yenilmez, J. Kong, H. Dai, Appl. Phys. Lett. 2001, 79, 3155; and A. Ural, Y. Li, H. Dai, Appl. Phys. Lett. 2002, 81, 3464. However, these approaches require pre-deposition and pre-patterning of nanoscale catalyst particle assemblies. Also, the probability of nanotubes bridging across different catalyst islands is difficult to predict and control. Several articles also note that nanotubes can be aligned in horizontal configurations through electric fields or microfluidic forces. See A. Star, et al., Angewandte Chem. International Edition 40, 1721 (2001) and T. Rueckes, et al., Science 289, 94 (2000). However, these methods are also complicated, and are difficult to scale (e.g., create them reproducibly on an 8 inch Si wafer) and to control, for developing devices for applications. Moreover, in these cases the nanotubes are not rooted to the substrate (i.e. they are just lying on them, and hence not very robust).
Several methods have also been suggested for controlled placement of carbon nanotubes onto electrode pairs, including AC bias-enhanced deposition and chemically modified adsorption. See L. A. Nagahara, I. Amiani, J. Lewenstein, R. K. Tsui, Appl. Phys. Lett. 2002, 80, 3826 and M. Burgard, G. Buesberg, G. Philipp, J. Muster, S. Roth, Adv. Mater. 1998, 10, 584.
Some of the present inventors have also previously suggested to selectively grow carbon nanotubes on silica templates located on a silicon substrate without growing the nanotubes on the silicon substrate. See Z. J. Zhang, B. Q. Wei, G. Ramanath, P. M. Ajayan, Appl. Phys. Lett. 77, 3764 (2000). The use of this template structure is advantageous in that it does not require the deposition and patterning of the catalyst material. However, as can be seen in FIG. 4 of the Z. J. Zhang et al. article, while roughly vertical and horizontal nanotubes were simultaneously grown on the template structures, it was not possible to controllably align nanotubes during growth in a direction perpendicular to the silica template structure surfaces. For example, as shown in the insert in FIG. 4 of this article, the nanotubes were not aligned precisely and controllably.